Nonwoven structures formed from a variety of materials, including natural and synthetic fibers in both staple and continuous form, have long been known and used in depth filter operations. Such depth filters generally have a range of pore diameters. If the filter medium is thin, the larger particles in the fluid being filtered will pass through those areas having the larger pores. If the effluent passing through the filter medium is then passed through a second equal layer, some of the larger particles remaining in the fluid will be removed as they encounter more finely pored areas. Similarly, use of a third equal filter layer will remove additional large particles, further increasing the filtration efficiency. Use of a thick layer of filter medium will have the same effect as using multiple layers of equal total thickness. The increased efficiency so obtained is one of the motivations for using depth filtration.
To be useful for a given application, a depth filter must provide the requisite level of efficiency, that is, an acceptable level of removal of particles of a specified size present in a fluid being filtered. Another important measure of the performance of a filter is the time to clogging in a given type of service, that is, the time at which the pressure across the filter has either reached a level at which an undesirable or unacceptable power input is required to maintain adequate flow, or the potential for filter collapse with the accompanying loss of integrity and effluent contamination is too high.
To extend filter life, it has long been the practice to design depth filters such that their density is lower in the upstream portions, thus providing relatively larger pores upstream and smaller pores downstream. By virtue of the graded density, the contaminated fluid passes through progressively smaller pores, and particulate material being filtered from the incident fluid penetrates to varying depths according to its size, thereby allowing the filter element to accomodate more solids (a higher dirt capacity) without affecting flow and consequently providing a longer effective life for the depth filter. Stated otherwise, in theory, the larger upstream pores remove larger particles which would otherwise clog the downstream, finer pores and filter life is thereby extended.
The density of the filter medium is, however, in itself, an important determinant of the medium's behavior in service. The optimum density of a filter medium is determined by two factors:
(1) In order to have a high dirt capacity, the percent voids volume in the depth filter should be as high as possible. The reasons for this may be seen by comparing a gravel screen made using woven wire with a metal plate equal in size to the screen but containing a single hole. The metal plate will be clogged by a single oversized particle, while the screen, requiring a large number of particles to become clogged, will remain in service longer.
(2) In a fibrous depth filter, there is an upper limit beyond which further increasing the percent voids volume becomes undesirable. As the voids volume is increased, the fibrous depth filter is more readily compressed by the pressure drop generated by the fluid passing through it; this is particularly troublesome when the fluid is viscous where, if the percent voids volume is too high, the filter medium will collapse at a very low differential pressure. As it collapses, the pores become smaller and the differential pressure increases, causing still more compression. The resulting rapid increase in pressure drop then tends to reduce life rather than--as might otherwise be expected with a high voids volume filter--extending it. Use of a very low density (high voids volume) can also make the filter very soft and thereby easily damaged in normal handling.
Thus, there is a practical upper limit to voids volume, the value of which depends on the clean differential pressure at which the filter is to be used. For any given type of service there will be an optimum percent voids volume at which filter life will be at a maximum.
As noted above, attempts have previously been made to provide depth filters from fibrous materials and to extend their effective life by providing a graduated porosity, accomplished by a density profile with the density increasing in the direction of flow of the fluid being filtered. These attempts have met with some success but the filter structures have substantial limitations. These include relatively short life due to the limited range through which pore diameters can be changed, and reduction in pore diameters due to compression when used with viscous fluids or at very high liquid flow rates.
The subject invention, then, is directed to cylindrical fibrous structures, particularly useful as depth filters, and a method of manufacturing them which substantially overcomes the shortcomings of the cylindrical fibrous depth filters which have heretofore been used. As will become apparent from the following description of the invention, the cylindrical fibrous structures of this invention typically have, relative to fibrous cylindrical depth filters of the type previously available, extended filter life, i.e., higher dirt capacity at equal efficiency, or better efficiency at equal life, or both better efficiency and higher dirt capacity. They also have the ability to remove much finer particulate contaminants than have heretofore been capable of being removed by previously available commercial fibrous cylindrical depth filters.